Adam Christensen

Thermal transport properties of thin films of small molecule organic semiconductors

A series of harmonic Joule-heating experiments have been employed to determine the thermal conductivities of thin films of pentacene, N,N′-diphenyl−N,N′-di(3-methylphenyl)−(1,1′-biphenyl)-4,4′-diamine, and tris(8-hydroquinolinato)aluminum, three widely used organic semiconductors. Room-temperature thermal conductivity values of 0.51, 0.24, and 0.48W∕mK were measured for films of these three compounds, respectively. These values are over two orders of magnitude lower than those of inorganic semiconductors. While amorphous films were found to display only small thermal conductivity changes over the temperature range of 228–350 K, pentacene exhibited stronger variations that are typical of phonon-phonon scattering observed in polycrystalline semiconductors.

Multiscale Lattice Boltzmann Modeling of Phonon Transport in Crystalline Semiconductor Materials

A coupled lattice Boltzmann (LB)–finite-difference (FD) method is used to solve for the heat transport in a two-dimensional domain. The LB method is used to capture relevant phonon physics near a microscopic heat-generation region by solving the Boltzmann transport equation, while a finite-difference model is used to capture the thermal transport at the macroscopic level. The coupling region between the LB and FD domains, which enables multiscale modeling, is discussed. The model is evaluated versus other numerical methods as well as experimental results. In all cases, the multiscale approach yielded results that were accurate to within the experimental uncertainty.

Micro-Raman thermometry in the presence of complex stresses in GaN devices

Raman thermometry is often utilized to measure temperature in gallium nitride (GaN) electronics. However, the accuracy of the technique is subject to errors arising from stresses which develop during device operation as a result of both thermoelastic and inverse piezoelectric effects. To assess the implications of these stresses on Raman thermometry, we investigate the use of the Stokes peak position, linewidth, and Stokes to anti-Stokes intensity ratio to estimate the temperature of GaN devices during operation. Our results indicate that only temperature measurements obtained from the intensity ratio method are independent of these stresses. Measurements using the linewidth, meanwhile, were found to correspond well with those obtained from the intensity ratio through the use of a reference condition which accounted for the stress dependency of this spectral component. These results were then compared to a three dimensional finite element model which yielded a correlation to within 5% between the computat...

Thermal Management Methods for Compact High Power LED Arrays

The package and system level temperature distributions of a high power (>1W) light emitting diode (LED) array has been investigated using numerical heat flow models. For this analysis, a thermal resistor network model was combined with a 3D finite element submodel of an LED structure to predict system and die level temperatures. The impact of LED array density, LED power density, and active versus passive cooling methods on device operation were calculated. In order to help understand the role of various thermal resistances in cooling such compact arrays, the thermal resistance network was analyzed in order to estimate the contributions from materials as well as active and passive cooling schemes. An analysis of thermal stresses and residual stresses in the die are also calculated based on power dissipation and convection heat transfer coefficients. Results show that the thermal stress in the GaN layer are compressive which can impact the band gap and performance of the LEDs.

Heat dissipation in high-power GaN electronics on thermally resistive substrates

The heat dissipation in GaN devices grown on low thermal conductivity lithium gallate (LGO) substrates was investigated. The thermal conductivity of single-crystal LGO was measured utilizing the 3/spl omega/ technique for temperatures ranging from 100 K-500 K. For the GaN layer, the thermal conductivity was estimated using a phonon transport model which included dislocation density and temperature dependence. These data were then used in a finite element program to determine the thermal behavior of a heterojunction field-effect transistor. Based on a maximum junction temperature of 500 K, it was found that devices with a power dissipation of 1 W/mm were possible if the primary heat dissipation path was through the low thermal conductivity substrate. However, in using a front side cooling scheme, results suggest that it may be possible to develop devices with power dissipation in the range of 10 W/mm.

Assessment of stress contributions in GaN high electron mobility transistors of differing substrates using Raman spectroscopy

The capability of gallium nitride (GaN) high power transistors arises, in large part, due to piezoelectric polarizations that induce the formation of a carrier rich two-dimensional electron gas. These polarizations, in turn, are directly related to the strain and hence stress that is present within the transistor. As a consequence, the stress load, as well as its measurement, is extremely important to the optimization of this device class. In response, this study demonstrates a technique to quantify the magnitude of operational thermoelastic stress that evolves in a GaN transistor through simultaneous use of the Raman signal’s Stokes peak position and linewidth. After verifying the technique through comparison with a finite element model, the method is then utilized in the analysis of high electron mobility transistors grown on silicon (Si) and silicon carbide (SiC) substrates. For each series of device, the major stress contributors—thermoelastic, converse piezoelectric, and residual—are acquired and com...

Temperature- and Doping-Dependent Anisotropic Stationary Electron Velocity in Wurtzite GaN

The influences of temperature, dopant density, free-carrier density, and field orientation on the electron drift mobility and velocity-field relationship in wurtzite c -axis GaN are quantified by means of theoretical investigation. Electron velocity perpendicular to the growth plane is uniformly lower than that parallel to the growth plane for field strengths below 500 kV/cm, although anisotropy within the basal plane itself is found to be insignificant. The calculated low-field electron mobility is demonstrated to be consistent with recent Hall measurements over a range of dopant densities. Low-field mobility is enhanced under the influence of free-carrier densities above the background doping due to both increased screening of ionized impurities and a reduction in A1-LO phonon lifetime through the plasmon-phonon interaction.

Lattice boltzmann and discrete ordinates methods for phonon transport modeling: A comparative study

The description of heat transport at small length scales is very important in understanding a wide range of micro and nanoscale systems. In systems where coherent phonon transport effects are negligible, the Boltzmann transport equation (BTE) is often employed to describe the distribution and propagation of thermal energy in the lattice. The phonon distribution function depends not only on the temporal and spatial coordinates, but also on polarization and wave vector, making fully-resolved simulations very expensive. Therefore, there is a need to develop accurate and efficient numerical techniques for the solution of the BTE. The discrete ordinates method (DOM) and more recently, the lattice Boltzmann method (LBM) have been used for this purpose. In this work, a comparison between the numerical solution of the phonon BTE by the LBM and DOM is made in order to delineate the strengths and weaknesses of these approaches. Test cases are chosen with Knudsen (Kn) numbers varying between 0.01–100 to cover the full range of diffusive to ballistic phonon transport. The results show that solutions obtained from both methods converge to analytical results for the 1 dimensional phonon transport in a slab. Solutions obtained by two methods converge to analytical solutions of 2 dimensional problems at low Kn. However, solution accuracy is strongly determined by angular resolution for moderate to high Kn. Since the number of propagation directions in LBM are limited, significant errors are engendered in multi-dimensional acoustically-thin problems. DOM also suffers errors at low angular resolutions for high Kn, but yields accurate solutions when sufficient angular resolution is employed.Copyright

Thermal design considerations in the packaging of GaN based light emitting diodes

The temperature distribution of a dual Multi-Quantum Well (MQW) light emitting diode (LED) has been investigated using both infrared imaging and micro-Raman Spectroscopy; mean values over the device yielded temperatures ranging from 30-75°C. The InGaN/GaN based LED, grown by Metal Organic Chemical Vapor Deposition (MOCVD), was also studied using the 3ω method in order to determine an effective thermal conductivity of the MQW stack in the temperature range from 300-540K. The LED structure under investigation showed effective thermal conductivities in the range from 82-140 W/mK with the peak conductivity occurring at 440K, well above room temperature. Using temperature dependent properties determined experimentally, a numerical model of the LED structure was developed in order to study the effect that the package thermal resistance and input power has on the temperature of the device.

Heat Dissipation in GaN Power Semiconductor Devices

In this work, a numerical study is presented of the impact of growth substrates on thermal dissipation in GaN devices. Substrates included in this study are sapphire, SiC, GaN, ZnO, and LiGaO2 . Based on a model high power HFET device with the rear side held at a fixed temperature, the maximum junction temperature in the devices were calculated using finite element analysis and compared. Both interface resistance and the effects of dislocations in the GaN layer were accounted for. Results show that state of the art devices dissipating 10 W/mm of power must be fabricated on high thermal conductivity substrates like GaN or SiC when rear side heat dissipation is utilized. In contrast, an analysis of high heat flux removal convective cooling was investigated for the application of front side heat dissipation. These results show that junction temperatures below 150°C are readily obtainable using this method and are substrate independent. The implications of the substrate independent cooling scheme are discussed.Copyright